Hi Nugo, thanks for contributing!
I'm afraid the "Model 7" you describe belongs to the former Northrop (Douglas El Segundo) numbering system. Apart from models 7 and 8, I'm not quite sure about how the models 1 to 6 were allocated, but they correspond to the following:
Alpha 1, 2, C-19, 3, 4, 4A
Beta 3, 3D
Delta 1A, 1B, 1C, 1D, 1D Executive, 1D-3, 1D-7, 1D(-8), 1E (= Gamma 1E)
Gamma 2A, 2B, 2C (YA-13), 2D, 2D2 (2H), 2E, 2EC, 2ED, 2ED-C, 2F, 2G, 2J-2, 2L
Gamma 3A (XP-948)
XFT-1
Gamma 5A, 5B, XA-16, A-17, A-17AS, 5D, BT-1, BT-2, Douglas 1X
XBT-1
7A (attack proposal), (DB-)7B (Havoc prototype)
Gamma 8A (A-17A), 8A-1, 8A-2, 8A-3N, 8A-3P, 8A-4, (DB-)8A-5 (A-33)
Now how the DB-19 ("BT-1S") fits in there is mysterious, unless it's a very old typo for DB-9!!
Thanks anyway!